More than 8,000 Fairfax County teachers attended a back-to-school rally yesterday at which Superintendent Robert R. Spillane attempted to calm their anxieties about the new pay-for-performance program that is being adopted this year.

Spillane told the teachers the more rigorous evaluation system upon which merit pay will be based is a necessary risk to improve education and a required tradeoff for the hefty raises -- nearly 30 percent over three years -- that teachers have been promised.

"Salary is only the first step," said Spillane. "Salaries must be wedded to an evaluation system that ensures quality."

The rally, which took place at George Mason University south of Fairfax City, was the second held by the school system, whose 131,000 students report for classes next week.

The merit pay plan is the first of its kind in the Washington area and has won the school system national recognition. But, like pay-for-performance plans elsewhere in the nation, it has produced widespread concern among teachers about fairness.

Forty percent of the county's teachers will be rated this year under the new on-the-job evaluation system that will be the basis for setting merit pay. The system was tried in eight schools last year.

By 1989-90, the evaluation system will be used for setting the pay of every teacher in the county. All teachers will be rated under the new five-step system, and those with more than six years of experience may apply for 10 percent merit pay bonuses that require an even more strenuous evaluation.

The average teacher salary in Fairfax last year was nearly $31,000, and teachers received an average 12 percent raise in July, the first part of the three-year salary increase.

Spillane acknowledged that teachers are anxious about the program, and promised that some procedural problems would be ironed out. Contrary to rumor, he said, there are no quotas on the number of teachers who may apply for and win bonuses.

Teachers interviewed after the two-hour rally, which included satirical skits, music and a speech by county teacher of the year Ron Axelrod, said they were heartened by Spillane's comments.

"Teachers have a lot of anxieties, and he did a good job recognizing some of those anxieties," said Sherry Reynolds, a government and economics teacher at Lake Braddock Secondary School.

"When you tell teachers that you understand their problems, you get to them," said Fairfax High government teacher Diane Scheurich.

Leaders of the county's two teachers unions differed in their reaction to Spillane's speech.

Mimi Dash, president of the 6,600-member Fairfax Education Association, which has endorsed the pay-for-performance plan, said "the rumor mill has been the problem" dogging the plan from the start and that Spillane was wise to address it.

"Morale will be an issue and he's going to have to play a major role in making sure {merit pay} is implemented fairly," she said.

But Rick Nelson, whose 900-member Fairfax County Federation of Teachers opposes the plan as inherently unfair, said Spillane's attempts at reassurance would do little good.

"They can't judge a person's teaching in six hours, and teachers know that," Nelson said.